Recently, the needs for secondary batteries (batteries) have been increasing more and more. For example, in the field of automobiles, since many electric devices which are operated by receiving power from a secondary battery are mounted, the needs for car-mounted secondary batteries are high. As a requirement for secondary batteries, for example, it is required to always maintain a secondary battery in a usable state. In order to satisfy such a requirement, for example, a device or the like which monitors the state of the secondary battery has been conventionally developed.
As an example of the state monitoring of a secondary battery, Patent Document 1 discloses a method of estimating the opened-circuit voltage and remaining capacity of the secondary battery by approximating transient changes of the secondary battery separately by three different time constants. Like this one, many techniques for detecting the charge rate (SOC: State of Charge) and degree of deterioration (SOH: State of Health) of the secondary battery have been conventionally known. Furthermore, in order to maintain SOC of the secondary battery, for example, for a car-mounted secondary battery, a technique of controlling charging by an alternator is known.
A secondary battery has an upper limit on chargeable electric capacity, and SOC of this upper limit is called SOC of a chargeable limit. Even if charging is further continued for the secondary battery that has SOC of the chargeable limit, charge is not carried out almost at all, and energy is wastefully consumed. Also, in an automobile, load on an engine is unnecessarily increased, and fuel consumption is worsened.
Therefore, in order to avoid such unnecessary charge and improve fuel consumption, charge control has been conventionally carried out for secondary batteries. Generally, in conventional charge control, charge of a secondary battery is controlled so that SOC has a predetermined value or be within a predetermined range. The value or the range of SOC serving as a condition for carrying out the charge control is fixed to a constant value or a constant range set in advance.
A conventional method of charge control will be explained by using FIG. 3. In order to explain the conventional charge control method, FIG. 3 shows an example of transition of SOC when charge control is carried out. In the control of the charge rate of a battery, a charge current is controlled (partial charge-rate control) so that SOC is maintained in a predetermined charge control range (range between an upper limit SOC1 and a lower limit SOC2 shown in FIG. 3). When such charge control is carried out, unnecessary power generation such as charging at the upper limit SOC1 or higher can be eliminated, load on an engine can be reduced to improve fuel consumption. Moreover, since the charge is carried out so that SOC of the battery does not become equal to or lower than the lower limit SOC2, the charge state of the battery can be maintained in an appropriate state.